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19.2 Viruses Replicate Only in Host Cells Viruses do not have organelles to build proteins, replicate nucleic acids, metabolize, etc. Needs a host cell to do it. Viruses can only infect certain hosts (host range) Only certain species Only specific cells/tissues within species Ex: flu only infects cells of the respiratory system

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General Features of Viral Replicative Cycles Infection begins with the insertion of the viral nucleic acid into the host cell: Use tail fibers to inject DNA into host cell (like a needle) Endocytosis of entire virus Viral DNA commandeers cells to make more viruses using the cell’s own machinery. Kills cells when done.

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The Lysogenic Cycle Same beginning and end as lytic cycle Insert DNA …… make copies and cell lyses After the DNA is inserted, it is integrated into host cell’s DNA. Prophage; 1 gene codes for a protein that prevents transcription of the rest of its genes. Cell grows and replicates (binary fission) normally with viral DNA still “hiding” in host DNA. lytic cycle is induced or Phage DNA circularizes. Certain factors determine whether lysogenic cycle is entered Lysogenic cycle Prophage Daughter cell with prophage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Cell divisions produce a population of bacteria infected with the prophage. The bacterium reproduces, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage.

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Evolution of Viruses Not living but effects all living things. Evolved after life. Naked DNA infected injured cells at first, then evolved to have an envelope and could infect healthy cells. Viral nucleic acids are more similar to host’s nucleic acids but there are some viral genes that are similar for all viruses.